1. Field of the Invention
The present invention relates to a process for producing a nitride semiconductor light-emitting device, and more particularly to a process for producing a nitride semiconductor light-emitting device having a novel structure in which a transparent electrode layer for improving contact resistance on a p-electrode may be eliminated.
2. Description of the Related Art
A nitride semiconductor light-emitting device is a high output power optical device that produces short wavelength light such as blue, green light, or the like, thus enabling it to produce a variety of colors, and attracts a great deal of attention in the related technical art
A constituent material of the nitride semiconductor light-emitting device is semiconductor single crystals of a composition formula AlxInyGa(1−x−y)N (0≦x,y,x+y≦1) Nitride semiconductor single crystals may be grown on substrates such as sapphire or SiC by using crystal-growth methods such as MOCVD (Metal Organic Chemical Vapor Deposition), HVPE (Hydride Vapor Phase Epitaxy), and the like. These substrates for growth of nitride single crystals are electrically insulative, and thereby, electrodes cannot be mounted on the back surface of the substrate unlike conventional light-emitting devices. Thus both electrodes must be formed on a crystal-grown semiconductor layer. FIG. 1 shows the structure of a conventional GaN-based light-emitting device.
Referring to FIG. 1, the conventional nitride semiconductor light-emitting device; which is designated by reference number 20, includes a sapphire substrate 11, and an n-type nitride semiconductor 15, an active layer 16 and a p-type nitride semiconductor 17 formed sequentially thereon. Further, in order to improve a lattice matching of the n-type nitride semiconductor layer 15 with the sapphire substrate 11, a buffer layer (not shown) such as AlN, GaN or AlGaN may be formed prior to growing the n-type nitride semiconductor layer 15.
As discussed above, since the sapphire substrate 11 is electrically insulative, formation of both electrodes on the upper surface thereof may be achieved by etching the p-type nitride semiconductor layer 17 and the active layer 16, at a predetermined region, to expose a portion of the upper surface of the n-type nitride semiconductor layer 15 corresponding to the predetermined region, and forming an n-electrode 19a made of a Ti/Al layer on the upper exposed surface portion of the n-type nitride semiconductor layer 15.
However, the above-mentioned p-type nitride semiconductor layer 17 has a relatively high resistance, and so a layer capable of forming an ohmic contact with conventional electrodes is added thereto. Also, this layer needs to be formed of a highly light-transmissive material so as to prevent deterioration of light-emitting efficiency.
For this purpose, U.S. Pat. No. 5,563,422 (Applicant: Nichia Chemical Industries, Ltd, Japan, Issued on Oct. 8, 1996) proposes formation of a transparent electrode 18 made of Ni/Au to form an ohmic contact, prior to forming a p-electrode 19b on the p-type nitride semiconductor layer 17. The transparent electrode 18 may form an ohmic contact while increasing an application area of electric current, thereby lowering a forward voltage (Vf).
However, the transparent electrode 18 made of Ni/Au has a transmissivity of only about 60%, suffering from great loss of light due to absorption.
To overcome this low light-transmissivity problem, there had been proposed formation of a layer of ITO (Indium Tin Oxide), known as have a transmissivity of more than about 90%, in place of the Ni/Au layer. But, since the ITO has a weak adhesiveness to nitride crystals and also a work function of 4.7 to 5.2 eV, compared to 7.5 eV of a p-type GaN, direct vapor-deposition of ITO on the p-type GaN layer does not form an ohmic contact and thus an additional formation of a thin Zn- or C-doped Ni/Au layer is necessary.
Further, a complex process is required to add a layer for an ohmic contact of the above-mentioned transparent electrode layer. A heat treatment process is needed to form NiO2 with a predetermined transmissivity after forming the Ni/Au layer, for instance, resulting in disadvantages of a complex process.
To resolve this problem, a method may be considered which includes forming an n-type nitride semiconductor layer having a relatively lower electrical resistance on the upper part of an active layer, and forming a p-type nitride semiconductor layer having a relatively higher electrical resistance between the active layer and a substrate. But, this method is not suitable for the p-type nitride semiconductor because of a heat treatment process required to activate p-type impurities.